In many applications, a communication medium is shared among a number of nodes. The nodes compete with one another for access to the shared communication medium. At any given moment, there may be more than one of the nodes that wish to transmit data over the shared communication medium. A system is typically put in place to facilitate access to the shared communication medium by the various nodes. Various categories of such multiple access systems have been developed.
One category of multiple access systems utilizes contention protocols. Examples of these contention protocols include the ALOHA protocol and the slotted ALOHA protocol, which are known in the art. Here, each node is allowed to freely transmit its data over the shared communication medium at any time or any slotted time. In a system employing a hub, each node sends its transmission to the hub, which then broadcasts the transmission to all nodes. In a system without a hub, each node directly broadcasts its transmission to all nodes. In either case, every node listens to the channel for its own transmission and attempts to receive it. If a node is unsuccessful in receiving its own transmission, the node can assume that its transmission was involved in a collision with another transmission, and the node simply re-transmits its data after waiting a random amount of time. In this manner, collisions are allowed to occur but are resolved by the nodes.
Another category of multiple access systems utilizes carrier sense protocols. Examples include persistent carrier sense multiple access (persistent CSMA) and non-persistent carrier sense multiple access (non-persistent CSMA) protocols, which are known in the art. Generally speaking, these protocols require each node to listen to the shared communication medium before transmitting. Only if the shared communication medium is available is the node allowed to transmit its data. In persistent CSMA, when a node senses that the shared communication medium is not available, the node continually listens to the shared communication medium and attempts to transmit as soon as the medium becomes available. In non-persistent CSMA, when a node senses that the shared communication medium is not available, the node waits an amount of time before attempting to listen to the shared communication channel for an opportunity to transmit. Even though a node listens first before transmitting, there still exists a probability for collisions. This is because when the medium is available, two or more nodes can detect the availability and decide that they are going to transmit data. Various techniques have been developed to handle such collisions.
Yet another category of multiple access systems utilizes contention free protocols. Here, each node can reserve the shared communication medium in order to transmit data. The node can transmit data without colliding with transmissions from other nodes. This is because the shared communication medium is reserved, for a particular time duration for example, for the node's transmission and not for any other transmission. A significant advantage of contention free protocols is that the communication medium is not taken up by unsuccessful transmissions that collide with one another and the resulting re-transmission attempts. This can lead to a more efficient use of the shared communication medium, especially as the number of nodes and number of data transmissions increase.
However, contention free protocols require a reservation process that allows nodes to reserve use of the shared communication medium. Making such reservations also requires communications, which are also susceptible to collisions. Collisions within the reservation process can lead to collisions during subsequent data transmission. Thus, to take full advantage of the benefits of contention free access, more efficient systems for reservation of the shared communication medium are needed.